I had an aquarium for about an year and this week my mum just installed a heater in it. On asking, she told me that fishes would die in cold water.

But from Henry's Law, isn't it that more oxygen would dissolve in cold water and enabling fish to get more oxygen for Metabolism.

My research on this told that fishes are cold blooded animals, meaning they can change the temperature of their body to match the environment. It says, that warmer water can be deadly for most fishes, the reason being Henry's law.

$\begingroup$An aquarium should also have a bubbler, so dissolved oxygen shouldn't be a problem. The temperature of the aquarium should match the preference of the fish species. So tropical fish would like between 75 and 80 °F year round. In the northern hemisphere we're approaching winter, so indoor temperature may fall below that. Freshwater goldfish don't need a heater.$\endgroup$
– MaxWNov 4 '18 at 2:32

$\begingroup$Ah.. so the heater is to make the metabolism work in stable manner, so if you have multiple fishes, then what??$\endgroup$
– Lakshya SinhaNov 4 '18 at 4:13

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$\begingroup$I'm voting to close this question as off-topic because it's a matter of biology not chemistry.$\endgroup$
– MithoronNov 4 '18 at 19:48

$\begingroup$While this question does have a significant amount of biology content. I think it has a sufficient amount of chemistry question to be on-topic.$\endgroup$
– A.K.Nov 4 '18 at 20:10

1 Answer
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While some fish are very tolerant of water conditions, they are not infinitely adaptable, they are typically tuned to a limited window of conditions that includes salt concentrations (ionic strength), specific ion and organic compound concentrations (some heavy metals and ions are very toxic to fish), pH, and oxygen concentration, not to mention temperature and pressure. So make sure to consult with an aquarium specialist (perhaps a book or internet site will do) to find the right conditions for your fish.

Cold blooded animals such as fish cannot regulate their body temperature to any significant extent, and as metabolic processes tend to be tuned to a narrow temperature window, fish do best within that window, and tend to die outside of it.

FYI I killed my first pet fish (a goldfish) within 24h after placing it in untreated tap water. I got very upset and blamed my mom.

Back to topic: gas solubility typically decreases at higher temperature, as you rightly point out, and this can be a problem for instance in hot summers, often also due to a combination with other temperature-related factors. As you have had an aquarium for over a year, and presumably your fish have survived that long, they seem to be reasonably hardy cold water species (depending where you live) and I don't see a good reason to regulate the temperature unless something is expected to change such as a significant drop in the room temperature. However I'd prefer not to get involved in a dispute with your mom!